High frequency transmission network



Dec. 31, 1940. A. ALFORD 2,226,686

NIGH FREQUENCY TRANSMISSION NETWORK Filed Nov. 16, 1937 4 Sheets-Sheet 1 FIG 2 J fig //0 FIG. 4

INVENTOR I ATTORN EY Dec. 31, 1940. A ALFQRD 2,226,686

HIGH FREQUENCY TRANSMISSION NETWORK Filed Nov. 16, 1937 4 Sheets-Sheet 2 INVENTOR AND/FEW ALFORD BY ATTO R N EY Dec. 31, 1940. A, ALFQRD 2,226,686

HIGH FREQUENCY TRANSMISSION NETWORK Filed Nov. 16, 1957 4 Sheets-Sheet s ,i

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0, 8 Q 2 S 2 Q Q ,6 If! w ve //7j/46 Z INVENTOR A/VDAAEWAL FORD BY 3 73" ATTORNEY Dec. 31, 1940.

A. ALFORD 2,225,586

HIGH FREQUENCY TRANSMISSION NETWORK Filed NOV. 16, 1957 4 Sheets-Sheet 4 3 RCEVVR l004 1 f2 /,00/ Iva; TRANSMITTER f5 reams/ 4075 F I6. 13. v

ATTORNEY.

Patented Dec. 31, 1940 HIGH FREQUENCY TRANSMISSION NET- WORK Andrew Alford, New York, N. Y., assignor to Mackay Radio and Telegraph Company, New York, N. Y., a corporation of Delaware Application November 16, 1937, Serial No. 174,759

15 Claims.

My invention relates to reentrant networks for modifying the transmission characteristics of a high frequency transmission line and more particularly to the use of composite reentrant networks for effecting a filtering action in the line circuit, an impedance matching between sections of the line or other useful functions.

This invention constitutes a further development of reentrant network circuits such as disclosed in my application, Ser. No. 118,886, filed January 2, 1937, issued as Patent No. 2,190,131, on February 13, 1940. In the above referred to application, a reentrant network circuit was disclosed in which filtering action, phase modification and impedance matching at certain frequencies may be accomplished by suitably dimensioning and proportioning the parts of the reentrant loop circuit. In this application means are provided comprising an impedance or equivalent means connected across a point in such a reentrant network, whereby the network may be so. modified as to produce the desired characteristics even though the dimensions do not correspond exactly tothe requirements set forth in the prior application. Furthermore, by means of this invention the additional impedance across a point of a reentrant network may be used to afford a filter circuit for other frequencies than those upon which the reentrant loop by itself is effective.

A principal object of my invention is to provide a structure which may be used as a highly efiicient transformer, filter or other wave modifying device which is simple in structure and easily adjustable.

It is a further object of my invention to provide a reentrant network which may be readily adjustable for performing the desired functions.

Another object of my invention is to provide means for adjusting a reentrant network for a particular frequency by means of an adjustable impedance connected thereacross.

It is a further object of my invention to provide a composite reentrant network which will serve to simultaneously filter more than one wavelength, by effecting a particular construction of the apparatus.

It is a further object of my invention to provide a composite network which serves as an impedance matching transformer.

The invention may be more clearly understood by reference to the accompanying drawings, in Which- Fig. 1 illustrates one form. of the network using an open line as the impedance,.

Fig. 2 illustrates a modification of Fig. 1, using a closed line as the impedance element,

Figs. 3, 4 and 5 illustrate further modifications of the network,

Fig. 6 illustrates an application of a variable condenser for particular use with a concentric cable conductor,

Fig. '7 illustrates a circuit in which the reentrant loop network in accordance with my invention is used to obtain certain useful data,

Figs. 8 and 9 are curves plotted from test data obtained with a network similar to that shown in Fig. 7.

Fig. 10 illustrates a network used as an im pedance matching transformer, I

Figs. 11 and 12 illustrate useful applications of the double filter network,

Fig. 13 illustrates an application of a network in accordance with my invention as an insulating support, and

Fig. 14 illustrates an application as an insulating spreader.

Referring more particularly to the drawings, in Fig. 1, IIII represents a source of high frequency alternating current, I03 is a load connected over transmission line I05 to the source. At a suitable point in transmission line I05 is connected reentrant-network shown generally at I06. This network comprises three arms, I01, I08, I09, forming together a closed loop. It may be noted that arm I01 comprises a section of transmission line I05. Connected to this reentrant loop at a point H0 is an additional impedance III which may comprise an open-ended transmission line, as shown in this figure. Arms I08, I 09, join with transmission line I05 and arm I01 at points H3, H5, respectively. Arms I01, I08, I09 together form a reentrant loop circuit which if suitably dimensioned will serve as a wave filter or a Wave modifying device as fully described in my copending application referred to above. According to this invention, however, a filter network is provided which may be sharply tuned to provide the desired relationship for filtering or impedance transformation regardless of I the exact relationship of the arms With respectto the frequency. This is accomplished by connecting across the loop at a suitable point an impedance represented by the open line III.

In order to more clearly define this filtering action the arms I01, I08, I09 may be represented by 02, 61 and 03, respectively, in which 0 expresses the length of the various arms in terms of wavelength. The. length in Wavelengths of open transmission line I I I may be'represente'd by of a network fan 2 sin 2 (1) an A sin 0 sin 0 in which 01, 02 and 63 have the values given above and x in the wavelength of the energy being transmitted.

The value of 1 given from this equation has been verified by actual experiment and found to be substantially correct. As a practical example the following particular data is set forth:v

For example, if 62 is equal to 5 feet=26.9 at

14,710 kc., and 1 91:03:75 feet=40.35 at 14,710 kc.,

then by substituting these values in Formula 1 it is found that LL A and since A=approximately 6'7 feet l=13.7 feet.

In .actual practice, when using No. 6 wire spaced at 12 inches and the usual type of insulators at the end of open ended transmission line, a correction of .4 of a foot should be applied to the calculated value of l in order to take account of the end effect. Thus I comes out 13.3 feet, the length necessary to make the whole structure a filter for 14,710 kc. An experiment performed to verify this calculation has shown that this result is correct within the limits of accuracy to be generally expected.

Returning now to fundamental Equation 1, it can be seen that when B1+OZ+03=3SO the sine factor in the numerator reduces to zero and likewise when 01+0362=180 the co-sine factor in the numerator reduces to zero. Thus the value of Z should be zero when the sum of the three arms is equal to 360 or the sum and difference of the arms is equal to 180, as set forth above. However, when either 61 or 03 are equal to zero or a multiple of 180 the denominator reduces to zero and accordingly the solution of the equation reduces to an indeterminate quantity. It has been found, when this indeterminate value exists, that any value of impedance may be connected across point III'I without. in any way effecting the filtering action of the reentrant network as such with respect to its fundamental frequency.

Since any value of impedance may be connected across the point without disturbing the cut-off under the conditions outlined in the paragraph above, it is evident that the attached open section may be designed in accordance with Equation 1 as to produce a cut-off at any second desired frequency. Thus, if we take f as the original frequency, for which the network 91, 02 and 63 in itself produces a complete cut-off, and f2 at another frequency which it is desired to also out off, we may utilize Formula 1 for this purpose by evaluating 01, 02 .and 03 in terms of the wavelength of frequency f2 and. calculating a value of l which will produce the desired result. It thus is possible to construct the filter in these special cases so that an exact cut-off may be had attwo separate and distinct frequencies.

I If a closed line is used in place of the open line shown in Fig. 1, it is only necessary to add to or subtract from Z as calculated for the open line, an amount equal to 90 in terms of wavelength. Fig. 2 represents a network in accordance with this arrangement. In this figure the various elements corresponding to those in Fig. 1 are indicated by similar reference characters; however, at a point in line I I I equal to is inserted a short-circuiting bar 2II forming a closed line. Since the short-circuit is at a termination equivalent to a quarter wave transmission .line the network may be grounded at this point and thus will serve as a means for grounding other frequencies introduced on the line.

Although in Figs. 1 and 2 I have shown the reentrant network in the form of a triangle and the impedance in the form of open or closed line sections, it should be understood that since the impedance may take other forms, an impedance element such as indicated by the inductance at 3II of Fig. 3, may be used to replace the line section. This impedance should be so formed and adjusted as to produce the equivalent effect of the open or closed sections as indicated above. It is clear that any suitable desirable form of impedance may be substituted for the transmission line section so long as the equivalency of the impedance is maintained.

In Fig. 4 I have shown the network in the form of two semi-circular lines connected to the transmission line. It is clear that in this case the arms 40B, 409 may be measured to the point M0 in a manner similar to that explained in detail in connection with Fig. 1. It is furthermore pointed out that the particular form and shape of the reentrant network is not material since it is only necessary that the proper relation among the several arms thereof be maintained. In each instance the lengths 01, 02 and 63 are to be measured along the conductors forming the various arms of the loop.

In Fig. 5 I have illustrated a condenser connected across point 5| 0 in place of the impedances shown in the other figures.

If 01+03+02 is made nearly equal to 360 and 01 and 03 are neither equal to 180 nor zero, then the right hand side of Equation 1 is quite small. Moreover, if 01+03+0z is slightly less than 360 while 01-l-63-02 is such that co-sine is small and is positive so that the length of the open ended section attached at point 5I0 is small. That means that the network may be brought into condition of exact cut-ofl. by connection of a small capacity across the line at 5I0. Thus a small condenser such as 5| 9 may be connected across the loop. This condenser may be made adjustable for either changing the frequency of cut-off of the-network or for tuning the network to cut-off at the fixed desired frequency. This type of construction is very useful since it is somewhat easier to adjust a condenser than to vary the length of a transmission line. This is particularly true in certain cases where the adjustment of connections tothe circuit is unusually difiicult.

All

In Fig. 6 is shown an application of this principle to a network made from lengths of. concentric transmission line. In this figure, 60.5 indicates the transmission. line to which is' joined at points 613, 6 l a reentrant loop formed of another concentric line conductor. This loop may bedesigned to closely as possible satisfy the relation 01+02+03=360. However, if, by some slight miscalculation, this value is not quite correct, compensation for the error" may be made by use of a condenser plate 62| connected to the outer concentric conductor and adjustable by any suitable means such as thumb screw 623, to and from the central conductor for varying the capacity to eifect an exact frequency cut-oil. This means maybe used for ready adjustment of a filter circuit and provides a means for easily making necessary or desired changes in tuning. This is particularly useful in concentric conductor circuits since it is quite diificult to readily change the connections at such points as 613, M5 to effeet the desired adjustment without use of the additional impedance.

This feature of easy adjustment is very useful when a network such as shown in Fig. 6 is employed for stabilizing or substantially controlling the frequency of an ultra-high frequency oscillator. In this case, the concentric tube may have to be maintained at a constant temperature and for that reason may be either wrapped with a layer of insulating material or with a wire through which heating current may be passed, or it may merely be well insulated and placed in a compartment maintained at a constant temperature. In any such event, however, access to the network is pretty much obstructed so that the length of the elements cannot easily be changed. By use of the thumb screw adjustment described in connection with Fig. 6, a means may be provided for easily completely tuning the circuit to the precise desired frequency.

The application of composite reentrant network as a filter circuit has been fully described above in connection with Figs. 1 to 6, inclusive. However, this network is capable of a number of other uses as will bemore fully set out in a particular consideration of Figs. 7 to 13, inclusive.

Referring now to Fig. 7, I20! represents a source of high frequency energy. Connected to this source is the transmission line I 202 terminated by impedance load I203. Across the transmission line is bridged a composite reentrant network I284 comprising the reentrant network A, B, C, A and the additional terminal impedance of length I lettered CD in the drawings. If this network is chosen so that AC equals CB, then it has been found that with certain adjustments of the length of the line CD, various effects of the standing wave distribution may be observed with changes in the length of the additional transmission line CD. It should be noted that in this figure the distance AB corresponds to the dimension 62 of Fig. 1, while AC corresponds to the dimensions 01 and CB to the dimension 63 of that figure.

With the apparatus set up as outlined in Fig. 7 and with the transmission line CD connected at a point across ACB such that AC equals CB, a series of experiments were made. The apparatus 7 was set up with particular dimensions of ACE in terms of the wavelength and the length of CD the maximum and minimum values of the standing wave. Likewise, measurements were taken of the distance it of the standing wave minimum from the point of junction of the reentrant network. These measurements were made for three different dimensions of network, as follows: (1)- With ACB equal to substantially .153 wavelength and the distance AB equal to substantially .93 wavelength. (2) With ACB equal to substantially .153 wavelength and the AB equal to substantially .056 wavelength. (3) With ACB equal to substantially .112 wavelength and AB equal to substantially .056 wavelength. From the data obtained by these measurements curves illustrated in Figs. 8 and 9 were obtained. The curves of Fig. 8 illustrate the change in the standing wave maximum to minimum ratio designated by Q, as abscissa plotted against the length l of the added impedance measured in the wavelength as ordinates. 'From these curves it is evident that not only does the standing wave ratio vary with changes in length Z but likewise for each such composite network there is a point in which the standing wave ratio obtained is equal to 1. latter feature constitutes a valuable principle for certain uses that may be made of the network.

In Fig. 9 the curves I, 2 and 3 illustrate the distance it, that is the distance of the standing wave minima from the network in terms of wavelength, as abscissa, against the length I of the impedance transmission line in wavelengths as ordinate.

From these curves it is evident that the position.

of the standing wave with respect to the network introduced thereby varies with adjustment of the short circuiting bar D, as well as with changes in dimensions of the network itself.

In accordance with the reciprocity law of high frequency networks, any network which can be used to introduce standing waves into a transmission line which is terminated in a surge impedance, may be used to match a transmission line normally having standing waves thereon. It is clear that this network produces a useful system to be used in impedance matching of transmission lines. For example, if a transmission line has a standing wave ratio of two, a network may be designed to match this impedance, in accordance with my invention. Taking,for example, the standing wave ratio Q equals 2, as stated above, a network may be devised in accordance with the above disclosure so that it will suit anyv or distance it may then be obtained from the curves illustrated in Fig. 9. It is then only necessary to measure the standing wave distribution of a line and locate the network at a distance spaced from the standing wave minimum indicated by the curves in Fig. 9, and to add thereto a line section of the proper length l as found from the curves of Fig. 8.

Although I have illustrated in Figs. 8 and 9 only three sets of curves corresponding to three specific dimensions of the network, it is clear that similar curves may be obtained for any desired dimension of network and thus the matching of a transmission line may be obtained, using any particular desired type of network. It is further evident that combinations of this matching feature with the filtering features as outlined in the paragraphs above, may be used so as to produce'both filtering and impedance matching simultaneously for diiferent frequencies.

In Fig. 10 is illustrated a system in which the This network in accordance with my invention is used as an impedance matching device. In this figure 'IOI constitutes a high frequency A. C. source and I02 is a load connected thereto. An impedance network 106 designed in accordance with the method outlined above is included in transmission line I05. The impedance unit is thus so designed that the impedance looking into the device at point H3 is equal to the surge impedance of the transmission line I05. Thus, this network is properly located on the transmission line in accordance with some such curves as those illustrated in Fig. 11 and thus no standing waves will remain on the portion of the line shown to the left of network I05.

It is clear that any number of such impedance devices could be used in a transmission line for the purpose of properly matching the impedance thereof and any desired combination of such composite networks may be used to secure the desired composite results. f

In Fig. 11 is illustrated by way of example one system using the double frequency filter circuit described in the earlier paragraphs of this specification. In this figure, 90I represents a source whichmay be a receiving antenna and 902 represents a load which may be a radio receiver. The receiving circuit 90I, 902 may be located in the vicinity of two transmitters operating at frequencies other than that to be received, as indicated at 900, 903. In this arrangement difficulty may be encountered in receiving the desired signals on EDI and receiver 902 due to the strong signals induced therein by reason of the proximity of the two transmitters. In this case, use may be made of the several filtering features as disclosed above. For example, if the desired frequency is f1 and the frequencies being transmitted close by are f2 and is the double frequency filter 904 may be constructed so as to present substantially complete cut-off at frequencies f2 and In Fig. 12 is shown an application of the double frequency filter to an antenna system in which a single antenna IOI is used to transmit two frequencies f2 and is from transmitters I002, I003, respectively, and to operate simultaneously to receive signals on receiver I004 at frequency ii. In this case, filters IOI2, IOI3, IOI4 are associated, respectively, with IBM, I003, I004. Filter IOI2 is tuned to exclude frequencies f1 and is while presenting substantially no impedance to f2, while filter IOI3 is tuned to exclude frequencies f1 and is while permitting passage of is, while filter IOI4 is tuned to exclude frequencies is and is while permitting passage of frequency f1. It is thereby made possible to operate upon a single antenna with these three frequencies without endangering interference of the signals in the different circuits.

It is clear that in Fig. 12, receiver I004 may be replaced by another transmitter operating at a third frequency or the transmitters I002, I003 may be replaced by receivers operating on the separate frequencies. It is thus made possible to operate simultaneously at different frequencies either in transmission or reception Without endangering mutual interference in the respective circuits.

The fact that with networks such as described above a design may be reached in which no standing waves are introduced into the line may be used in a somewhat different manner for completely and fully isolating a transmission line from its point of support. It sometimes occurs that in supporting transmission lines, particularly for transmission of high frequencies, difliculties arisel particularly in wet weather due to certain amount of leakage across insulators with a consequent introduction of additional losses into the line. In accordance with my invention it is possible'to use the network such as described above, to obviate this difiiculty.

- In Fig.13, by way of example, a filter network used for this purpose is illustrated. In this figure I30I represents a high frequency transmission line connecting together high frequency apparatus I302'and I306. This apparatus may be, for example, a high frequency transmitter, a high frequency receiver, or an antenna for either receiving or transmitting high frequency energy. Such apparatus as I302 is often mounted at a level different from another portion of the transmission line I30 I, for example, it may be mounted at a higher point. Because of this difference in level of mounting it is necessary to introduce a bend or angle in the transmission line, as indicated at I303. In order to introduce such an angle into the transmission line it is necessary that the line be fastened at I303 to some support, such as I304. If an ordinary support and insulators are used, the leakage across such insulators may introduce undesired additional impedances into the transmission line which may vary with changes in the atmospheric conditions, due to changes in leakage across the insulators. In order to avoid this loss, it is only necessary in accordance with my invention to design a network such as that shown in Fig. 7, and to so select the length lof the part CD as to cause the standing wave ratio to be equal to unity. Accordingly, since the network then will introduce no irregularities or reflectionsinto the line, the network mayserve to completely isolate this point from the surrounding support and therefore will introduce no undesired effect in the transmission line. This is clearly evident since with such an arrangement the short circuiting bar will be at some definite fixed point and it is accordingly immaterial what is connected to the transmission line below this short circuiting point. Networks used in this manner are shown in Fig. 13 at I305.

The insulating system described in connection with Fig. 13 may be used at the same time as a supporting means and a spreader if the parts of the reentrant loop are made of stiff conductors. An example of such an arrangement is shown in Fig. 14. In this figure the transmission line is indicated at I40I. At a point in the line where a desired support is required a network I405 is inserted. Thisnetwork may be dimensioned in accordancewith the disclosure above, so as to serve in place of an insulator, and is made of rods or tubing so as to give a sufiicient support. The network is preferably designed with a short circuiting bar I406, similar to that shown in connection with Fig. 2 to produce additional stiffness and to permit the lower ends of the network to be rigidly supported without adversely affecting the characteristics of the network. Although as stated above the rigid network may be designed to serve in place of an insulator, it is clear that a network serving as a filter or an impedance transformer, may likewise be made of a stiff material so as to serve simultaneously as a support and spreader 'for the transmission line conductors.

' While I have described particular embodiments of my invention for the purpose of illustration of various adaptations thereof, it is clear that other modifications may be made by one skilled in the art Within the spirit of the invention as defined in the appended claims.

I claim:

1.- A composite transmission modifying network foruse in a high frequency circuit, said network comprising a first uniform conductor, a second uniform conductor connected at its ends to the ends of said first conductor, and an impedance element connected to one of said conductors at a predetermined point intermediate the ends thereof.

2. A high frequency transmission circuit comprising a two conductor transmission line, 'a pair of conductors each of said conductors having its ends connected to spaced points on a corresponding conductor of said transmission line, said transmission line being continuous between said spaced points, and an impedance means connected to an intermediate point on said pair of conductors.

3. A high frequency transmission circuit comprising a two conductor transmission line, a pair of conductors each of said conductors having its ends connected to spaced points on a corresponding conductor of said transmission line, said transmission line being continuous between said spaced points, and an impedance means connected to an intermediate point on said pair of conductors, the sum of the lengths of said pair and the portion of the transmission line included between said predetermined points of connection being equal to 360 electrical degrees.

4. A high frequency transmission circuit comprising a two conductor transmission line, a pair of conductors each of said conductors having its ends connected to spaced points on a corresponding conductor of said transmission line, said transmission line being continuous between said spaced points, and an impedance means comprising a conductor pair connected to an intermediate point on said pair of conductors.

5. A high frequency transmission circuit comprising a two conductor transmission line, a pair of conductors each of said conductors having its ends connected to spaced points on a corresponding conductor of said transmission line, said transmission line being continuous between said spaced points, an impedance means comprising a conductor pair, and a short circuiting bar connected across said conductor pair at a predetermined distance from said intermediate point.

6. A high frequency transmission circuit comprising a two conductor transmission line and a filter network connected to said line comprising, a first conductor pair connected to said transmission line, a second conductor pair connected to said transmission line at a point spaced from the connection point of said first conductor pair, said first and second pairs being serially connected, and an impedance means connected at the junction of said serially connected pairs.

7. A high frequency transmission system in accordance with claim 6, in which said pair and the portion of the transmission line between spaced points are so dimensioned as to serve as a cutoff filter at one frequency and said impedance means together with said pair and the line is so chosen as to serve as a cutoff filter for a different frequency.

8. In a high frequency circuit, a uniform two conductor transmission line, a loop circuit comprising two conductors each of said conductors having both of its ends connected at each of their ends respectively to equally spaced points on a conductor of said transmission line, said transmission line remaining continuous between points of connection and an impedance means connected to predetermined points on said loop circuit conductors.

9. A high frequency circuit according to claim 8, in which the portions of said loop conductors between said predetermined points and said transmission line are equal and said impedance means comprises a section of two conductor line, dimensioned to produce a desired reaction in said line.

10. A high frequency circuit according to claim 8, in which said pair of conductors are relatively rigid, and said impedance means comprises relatively rigid conductors firmly attached thereto, and a short circuiting bar bridged across said conductors at a predetermined distance from said loop conductors.

11. In combination, a transmission line, a source of high frequency connected to one end of said line, a load connected to the other end of said line, and means connected to said transmission line intermediate the ends of said line comprising, a loop connected to two spaced points in said line, and an impedance means connected to an intermediate point on said loop for matching the impedance of the remainder of said line and said load to said source.

12. A method of matching the impedance of a load to a transmission line, using a reentrant loop circuit and an impedance coupled thereto which comprises, choosing the desired dimensions for said loop, measuring the ratio of the standing wave maximum to the standing wave minimum existing on said line, and the relative location of said maxima and minima along said line, determining the value of said impedance from said standing wave ratio and said desired dimensions, connecting said impedance across said loop at a predetermined point thereon, and connecting said assembled loop and impedance to said transmission line at a point determined by the value of said impedance and the position of said standing waves on said line.

13. In combination, a transmission line, extending in one direction, a second transmission line extending in a direction at an angle to said first transmission line and connected serially thereto, and means for connecting said transmission lines to a support at the junction thereof, comprising a conductor connected to one transmission line, a second conductor connected to said second transmission line, the other ends of said conductor being serially joined, and another conductor connected to the junction point of said first and second conductors, said conductors, the portion of said transmission line therebetween and said junction connected conductor being so dimensioned and arranged that they present substantially no impedance in the transmission line, and means for connecting the other end of said junction connected conductor to a support.

14. In combination, a high frequency apparatus designed to operate at a given frequency, a trans mission line connected to said high frequency.

apparatus, and means for keeping two other high frequency waves from interfering with said high frequency apparatus comprising, a pair of conductors connected to said transmission line at spaced points and serially connected together, said conductors and the portion of the transmission line between said points being so proportioned and arranged as to operate as a cut-01f respectively to said branch transmission lines and means to block the transmission of undesired frequencies from each said high frequency apparatus comprising closed loop conductors connected to said branch transmission lines and impedance elements connected to said loops at predetermined points thereon, said loop being designed to operate as a cut-off filter at one of said undesired frequencies and said impedance together with said loop being designed to operate as a cut-off 10 filter at another of said undesired frequencies.

ANDREW ALFORD. 

